Magnetic properties of CoFe2O4/CoFe2 bi-magnetic nanocomposites synthesized by reduction process using activated charcoal in argon atmosphere

Itegbeyogene P Ezekiel*, Thomas Moyo and Hafiz M I Abdallah

School of Chemistry and Physics, University of KwaZulu-Natal, P/Bag X5400, Durban 4000, South Africa

E-mail: 212562426@stu.ukzn.ac.za

Abstract. The core-shell architecture of CoFe2O4/CoFe2 nanocomposite was synthesized in a controlled flowing argon atmosphere by reduction reaction of CoFe2O4 nanoferrite with activated charcoal at 900 C. The parent CoFe2O4 nanoferrite was synthesized at 200 oC by glycol-thermal method and was subsequently followed by partial and full reduction to CoFe2O4/CoFe2 and CoFe2 respectively. Full reduction was achieved at a CoFe2O4 to C ratio of at least 1:6. The phase identification, morphology and magnetic properties of the parent sample and the nanocomposites were performed by X-ray diffraction (XRD), high resolution scanning electron microscopy, high resolution transmission electron microscopy, 57Fe Mössbauer spectroscopy and vibrating sample magnetometer. The parent sample had an average crystallite size of about 10 nm, XRD density of 5.41 g/cm3 and an average lattice parameter of 푎 = 8.382± 0.008 Å. The fully reduced CoFe2 compound had an increased crystallite size of about 65 nm. Its XRD density was estimated at about 7.9 g/cm3 based on bcc atomic arrangement of Co and Fe atoms with an average lattice parameter of 푎 =2.869±001 Å. The coercive field measured at room temperature decreased from 230 Oe to about 13 Oe after the reduction process. The nanocomposites show evidence of exchange coupling interaction and enhanced magnetizations with an increase in the energy product (BH)max from 0.143 MGOe for the parent sample to 0.528 MGOe for the bi-magnetic nanocomposites.

Keywords: Nano-ferrites; Nanocomposites, Bi-magnetic; Exchange interaction; (BH)max product; Reduction reaction

1. Introduction

CoFe2O4 ferrite has interesting properties due to its high crystalline anisotropy energy, moderate saturation magnetization, good chemical stability and wear-resistance [1-3]. It has an inverse spinel structure where the octahedral B-sites are occupied by Co2+ ions and half of the Fe3+ ions. The other half of the Fe3+ are contained in the tetrahedral A-sites of the spinel structure, aligned in parallel and in opposite direction to the moments of Fe3+ ions at the B sites. This leaves the magnetization and source of high anisotropy to the Co2+ ions in the B site [4]. On the other hand, the CoFe2 alloy is a soft ferromagnetic material with high saturation magnetization and low coercivity. Nanocomposites complexed by the exchange coupling interaction between hard and soft phases create interesting and

mutually compensating effects on the magnetic properties leading to enhanced magnetic properties such as the saturation magnetization and maximum energy product in the hysteresis loops [5]. The exchange coupling behavior of CoFe2O4/CoFe2 was studied by Leite et al [6] who used a parent CoFe2O4 sample synthesized by hydrothermal method with an initial crystallite size of 16 nm. Full reduction to CoFe2 was obtained at a ferrite to carbon ratio of 1:10. Nanocomposites of CoFe2O4/CoFe2 microfibers have also been studied by Liping et al [7] who used the sol-gel technique to synthesize the parent sample and performed the reduction reaction in a mixture of hydrogen-nitrogen gas environment. In the present work, we report on structural and magnetic studies of similar nanocomposites. We have used CoFe2O4 ferrite with smaller crystallite size. 2. Experimental details CoFe2O4 nano-ferrite was synthesized by glycol-thermal method from stoichiometric amounts of Sigma-Aldrich iron chloride FeCl3.6H2O (99%) and Cobalt chloride CoCl2.6H2O (98%) that was initially dissolved in deionized water. Different CoFe2O4/CoFe2 nanocomposites were produced by the reduction reaction of the parent sample with activated charcoal at 900 C for 3 hours in flowing high purity argon gas (99.999%) atmosphere in a Sentro-Tech Corp tube furnace. The samples produced and studied consisted of 1 mole of CoFe2O4 that was each reacted with 0.5, 1, 2, 4, 5, 6, 8, and 10 moles of carbon. The powder X-ray diffraction (XRD) patterns of the samples were performed on an Empyrean Analytical diffractometer type PW 1710 with CoKα radiation. The structural morphology of the samples was studied using Joel_JEM-2100 high-resolution transmission electron microscopy (HRTEM) and Zeiss high resolution scanning electron microscopy (HRSEM) The Mӧssbauer measurements were performed at room temperature using a convectional constant acceleration spectrometer with velocity calibration by an alpha-iron foil. Magnetic measurements were performed on a LakeShore model 735 vibrating sample magnetometer (VSM) at room temperature in applied fields of up to 14 kOe. 3. Results and discussion Figure 1 shows the XRD patterns of the as-prepared CoFe2O4 nanoferrite, sample annealed at 900 C and nanocomposites formed with 1, 2, 4, 6 and 10 moles of activated carbon. No impurity phases are detected in the parent and annealed samples as all peaks are indexed to the expected spinel structure (JCPDS no. 022-1086) associated with the Fd3m spatial group for CoFe2O4. The average lattice parameters, crystallite sizes and XRD densities for the pure phases are presented in Table 1. The values for the lattice parameter in the ferrite samples are close to the value of 8.3957 Å reported for bulk CoFe2O4 [8]. A slight increase in the lattice parameter for the annealed sample can be attributed to different distribution of metal cations and the surface distortions of particles as a result of increased crystallite size [9]. Figure 1 also shows the evolution of XRD patterns with increasing carbon used in the reduction reaction. Full reduction to CoFe2 was achieved at the ferrite to carbon ratio of at least 1:6 in the present case compared to 1:10 elsewhere [6]. These results show evidence of systematic reduction of the ferrite phase at the expense of the alloy phase. The XRD peaks for the CoFe2 phase have been indexed with respect to the Miller indices (110) and (200) corresponding to the body centered cubic (bcc) atomic arrangement. The crystallite sizes (D) were calculated using the Scherrer’s formula using the most prominent peaks (311) for CoFe2O4 and (110) for CoFe2. Figure 2 shows the evolution of the average crystallite sizes (D) for CoFe2O4 and CoFe2 components as a function of the initial carbon content of the nanocomposites. The point of intersection shows equal crystallite sizes of the hard to soft phases corresponding to a ferrite to carbon molar ratio of 1:2. This confirms the transition phase for the growth of CoFe2 phase on the CoFe2O4, thereby increasing the nanocomposite crystalline diameter. The morphology of the as-prepared sample is shown in Figure 3 (a) with no evidence of shell-like features on the surface. As one progresses through the images of the morphologies, it is clearly seen that the shells begin to develop all over the surface. However, this decrease, as the optimum ferrite to carbon molar ratio of 1:6 is approached. The fully reduced samples do not show a high degree of surface shelling.

Fig. 1. XRD patterns of CoFe2O4: (a) as-prepared (b), annealed at 900 C (c), reduced with 1C (d), 2C (e), 4C (f), 6C and (g) 10C.

Fig. 2. Crystallite size variation with carbon molar content.

Fig. 3. Morphology of CoFe2O4 + xC nanocomposites with carbon molar content: (a) 0x , (b)1x , (c) 2x , (d) 4x , (e) 6x and (f) 8x .

Table 1. Crystallite sizes (D), lattice parameters, (a) and X-ray densities XRD . Sample D (nm) a (Å)

XRD (g/cm3) ±0.02 ±0.005 ±0.02 Parent CoFe2O4 9.68 8.382 5.41 CoFe2O4 at 900 C 68.49 8.390 5.28 CoFe2 64.83 2.869 7.85

Table 2. Mössbauer parameters includes hyperfine fields, (Bhf), isomer shifts, ( ),quadrupole shifts, (∆EQ ), line widths, (LW), and fraction populations of Fe3+, (f ), of the parent sample and nanocomposites formed at 0.5, 2, 4, 8 and 10 moles of carbon.

C (mol)

Sub-pattern

Bhf (kOe) ±1.3

(mm/s) ±0.022

∆EQ (mm/s) ±0.019

LW (mm/s) ±0.04

f (%) ±2.3

0 A B

445.3 482.3

0.305 0.309

0.008 0.006

0.35 0.28

29.5 70.5

0.5 A B

469.0 489.7

0.744 0.268

0.057 -0.024

0.26 0.22

43.3 56.7

2 A B B2 B3

357.8 360.8 141.3 335.9

1.696 -1.504 2.156 -1.052

0.671 0.580 0.994 -1.199

0.30 0.23 0.31 0.23

26.4 30.1 25.2 18.3

4 A B B2 D

362.5 377.1 365.0 -

0.084 -0.588 2.347 2.000

-0.067 -0.369 0.043 -

0.32 0.11 0.19 -

41.4 13.2 11.6 33.4

8 A B

371.0 370.7

0.046 0.061

-0.150 0.112

0.25 0.20

42.0 58.0

10 A B D

367.5 382.8 -

0.027 0.029 1.315

0.002 -0.001 -

0.26 0.24 -

66.0 28.0 5.3

Mössbauer spectra for the parent sample and nanocomposites formed at 0.5, 2, 4, 8 and 10 moles of carbon are shown in Figure 4. The data was fitted by a minimum of two sextets and a doublet. The sextets are associated with Fe3+ ions at A-sites (tetrahedral) and B-sites (octahedral) of the spinel crystal structure [10]. A doublet was added in the analysis to account for the complex oxidation state of Fe atoms in the nanocomposites formed at 4 and 10 moles of activated carbon. The fitted parameters obtained are shown in Table 2. At room temperature, the parent sample has typical hyperfine field values at A-and B-sites of 445±1.3 and 482±1.3 kOe respectively for Fe3+. The obtained isomer shift values are in the range for Fe3+ [11], with the ∆EQ value nearly zero, which is a characteristic associated with Fe ion. Therefore this excludes the possible existence of Fe2+ in the parent sample. However, considering the nanocomposite at 0.5 moles of carbon, an increase in isomer shift values was observed on A-site, showing the evidence of Fe2+ [11], but B-site still retains in the Fe3+ range. Therefore, there is a resultant magnetic spin moment which can account for the enhanced magnetization observed at this mole ratio in VSM measurements. Magnetic hysteresis loops for the parent sample and the bi-magnetic nanocomposites are shown in Figure 5. The magnetic properties such as coercivity, (Hc), saturation magnetization (Ms) and remanent magnetization (Mr) can be deduced from the hysteresis curves. At room temperature we find Hc = 0.198 kOe, Ms= 32.8 emu/g and Mr= 2.6 emu/g was obtained for the as-prepared sample. An optimum saturation magnetization of 104.7 emu/g was observed at 6 moles of activated carbon. Excess of activated carbon was observed to reduce the saturation magnetization as seen in the hysteresis curve.

Fig. 4. Mössbauer spectra of parent sample and nanocomposites formed at 0.5, 2, 4, 8 and 10 moles of carbon.

In Figure 6 we show the variations of Hc, Ms, Mr, squareness ratio Mr/Ms, and maximum energy product (BH)max with mole ratios of activated carbon. Hc has a maximum value of 230 Oe at 5.0C moles and decreases to about 13 Oe after the reduction process. The Ms is observed to increase from 32.8 emu/g to 104.7 emu/g. The abrupt drop at 2 moles could be attributed to the complex covalence phase formed as confirmed in Mӧssbauer analysis. The optimum magnetization value was achieved at an optimum carbon ratio of 6 moles. An enhanced maximum energy product of 0.528 MGOe as against 0.143 MGOe of the as prepared sample was achieved at 0.5 moles of carbon, which suggests the most effectively coupled bi-magnet in our observation. This is a smaller value than that reported by Leite et al [6] (1.22 MGOe).using the same technique, with full reduction reported at 10 moles of carbon, but with bigger grain size of 16 nm parent sample. They suggested that the (BH)max depends on the magnetic properties of the precursor material. Our findings confirm this to be valid. Cabral et al [2], reported 0.6 MGOe, using hydrogen reduction technique, with a parent sample coercivity of 1.32 kOe and grain size of about 30 nm.

4. Conclusions Bi-magnetic nanocomposites of CoFe2O4/CoFe2 were successfully synthesized by reduction reaction. Full reduction was achieved with 6 moles of activated carbon for 1 mole of the ferrite. An enhanced maximum energy product (BH)max was obtained. Our results suggest that there is a critical crystallite size of the parent sample to achieve the best enhanced magnetic property and (BH)max for CoFe2O4/CoFe2 bi-magnetic nanocomposite. Excess carbon was observed to reduce the saturation magnetization due to the diluting effect of the carbon.

Acknowledgments We express our gratitude to the National Research Foundation (NRF), South Africa for the NNEP grant for the VSM equipment, EM unit (UKZN, WC) for HRSEM measurements and the Nigerian National Centre for Technology Management (NACETEM) for study leave (IPE).

Fig. 5. Hysteresis loops for CoFe2O4 and CoFe2O4/CoFe2 nanocomposites.

Fig. 6. Coercivity (Hc), saturation magnetization (Ms), squareness ratio (Mr/Ms), remanence magnetization (Mr), and maximum energy product (BH)max of CoFe2O4/CoFe2 at different mole ratio of activated carbon.

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